As far as I know, astronomy is generally an observational science. We see something and then try to explain why it is happening. The one exception that I know of is black holes: first it was thought of, then it was found.

Einstein's relativity is middle ground to me, he thought of light beams at the speed of light but obviously could observe gravity's effects.

Anyway, I guess my question is, what are the biggest discoveries that were thought of before they were seen in the sky?

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Off the top of my head, the Cosmic Microwave Background radiation was hypothesized as a consequence of Big Bang Theory before it was observed by accident by Penzias and Wilson. Also the light element abundances, also a consequence of BBT, was theoretical and is still being refined today through observations that supported the initial theory.

I don't know what your definition of "biggest discoveries" would be, but volcanoes on Io were theoretical before observed by Voyager 2. The discovery of the most volcanically active body in the solar system seems kinda big to me, but I am a planetary astronomer.

I think you're going to get multiple people posting several different things in response, so one of us responders should try to agglomerate the responses into a single reply.

Edited to add: Since you've accepted my answer over Andrew's I'll just append his to mine so it's more likely that people who just read the accepted ones will see it: "Neutron stars were predicted in 1934 by Baade and Zwicky, one year after the discovery of the neutron. They were not observationally confirmed until 1965 by Hewish and Okoye. It's hard to beat a prediction that sat around for 30 years before being confirmed."

Meh! I posted before I noticed that you'd beat me to it. I had the pleasure (well, mostly pain as I didn't do well) of taking a introductory course on stellar physics as UCSB from Stan Peale who was one of the Io guys. Wish I'd had that chance later, when I was more mature.
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dmckee♦Dec 6 '11 at 4:25

Actually, Pop III stars have not been observationally confirmed. They can't be until JWST based on current predictions. There are "Pop II.5" ultra-low-metallicity dwarfs, but these are not necessarily 1st-generation.
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keflavichDec 6 '11 at 17:00

Neutron stars were predicted in 1934 by Baade and Zwicky, one year after the discovery of the neutron. They were not observationally confirmed until 1965 by Hewish and Okoye. It's hard to beat a prediction that sat around for 30 years before being confirmed.

Philosophers of science, studying the extent and manner in which Einstein's Gen Rel became accepted by the scientific community, have noticed that the retrodiction of the anomaly in the perihelion of Mercury was considered more convincing than the prediction of the deflection of light, in part because everyone else had already tried to explain Mercury and failed.
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joseph f. johnsonFeb 12 '13 at 1:09

What comes to my mind are black holes. Their existence was postulated by Karl Schwarzschild in 1916, the theory was refined during the 1960s, but as to their existence, we still rely only on indirect evidence. There was a program to find flashes related to the final stages of the evaporation of a black hole started in 2008, but no hard evidence has been gathered so far. The Hawking radiation (if it truly exists) is too weak to be measured from Earth.

Dark matter is really neither a theoretical prediction nor an observationally supported empirical discovery. It's a cutesy name given to a discrepancy between mutually irreconcilable observations so that astrophysicists don't have to embarrass themselves at cocktail parties.
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AndrewDec 10 '11 at 17:15

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It's rather an example for the opposite - it has been observed (otherwisde nobody whould talk about it) but has not yet good theoretical explanation that fits the data.
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Arnold NeumaierMay 13 '12 at 9:26

@Andrew: It's actually 3 separate measurements which are irreconcilable with visible matter, which are all reconcilable using dark matter consistent with one another. This is a pretty firm empirical discovery. The three things are rotation curves, bound galaxy cluster velocities, and inflationary fluctuations. They all agree that Dark matter is more than visible matter by a factor of >3,5,5 respectively. You can't call it an embarassment with a made-up explanation when three separate methods all nontrivially give consistent answer.
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Ron MaimonJun 4 '12 at 19:26

Aristotle (4th century BCE) first hypothesized that the Earth is geographically divided into three types of climatic zones based on their distance from the equator (circles of latitude). In my opinion, it's the basis of one of the greatest science experiments of antiquity that never was. If the ancient Greek astronomers had been modern scientists in their methods, confirmation of the existence of geographical zones would have been the foundational experiment justifying spherical astronomy.

Another basic theoretical prediction that preceded observation is Earth's rotation, which I believe was first firmly predicted by Newton's celestial mechanics, and experimentally observed of course by Foucault's pendulum experiment in 1851.